add example for diff on first line

Author: fry69

tests/dummy/public/example.md

@@ -62,3 +62,22 @@ Router.map(function() {
 
 export default Router;
 ```
+
+With a diff on the first line: 
+
+```javascript {data-filename="app/router.js" data-diff="-1,+2"}
+import EmberRouter from '@ember/routing/router';
+import EmberRouter from '@embroider/router';
+import config from './config/environment';
+
+const Router = EmberRouter.extend({
+  location: config.locationType,
+  rootURL: config.rootURL
+});
+
+Router.map(function() {
+  this.route('about');
+});
+
+export default Router;
+```